11,573 research outputs found
The fractional integrated bi- parameter smooth transition autoregressive model
This paper introduces the fractionally integrated Bi-parameter smooth transition autoregressive model (FI-BSTAR model) as an extension of BSTAR model proposed by Siliverstovs (2005) and the fractionally integrated STAR model (FI-STAR model) proposed by van Dijk et al. (2002). Our FI-BSTAR model is able to simultaneously describe persistence and asymmetric smooth structural change in time series. An empirical application using monthly growth rates of the American producer price index is provided.Long Memory, Nonlinearity, Asymmetry, STAR models.
The Sloan Lens ACS Survey. IX. Colors, Lensing and Stellar Masses of Early-type Galaxies
We present the current photometric dataset for the Sloan Lens ACS (SLACS)
Survey, including HST photometry from ACS, WFPC2, and NICMOS. These data have
enabled the confirmation of an additional 15 grade `A' (certain) lens systems,
bringing the number of SLACS grade `A' lenses to 85; including 13 grade `B'
(likely) systems, SLACS has identified nearly 100 lenses and lens candidates.
Approximately 80% of the grade `A' systems have elliptical morphologies while
~10% show spiral structure; the remaining lenses have lenticular morphologies.
Spectroscopic redshifts for the lens and source are available for every system,
making SLACS the largest homogeneous dataset of galaxy-scale lenses to date. We
have developed a novel Bayesian stellar population analysis code to determine
robust stellar masses with accurate error estimates. We apply this code to
deep, high-resolution HST imaging and determine stellar masses with typical
statistical errors of 0.1 dex; we find that these stellar masses are unbiased
compared to estimates obtained using SDSS photometry, provided that informative
priors are used. The stellar masses range from 10^10.5 to 10^11.8 M and
the typical stellar mass fraction within the Einstein radius is 0.4, assuming a
Chabrier IMF. The ensemble properties of the SLACS lens galaxies, e.g. stellar
masses and projected ellipticities, appear to be indistinguishable from other
SDSS galaxies with similar stellar velocity dispersions. This further supports
that SLACS lenses are representative of the overall population of massive
early-type galaxies with M* >~ 10^11 M, and are therefore an ideal
dataset to investigate the kpc-scale distribution of luminous and dark matter
in galaxies out to z ~ 0.5.Comment: 20 pages, 18 figures, 5 tables, published in Ap
Closing in on the picture : analyzing interactions in video recordings
This paper provides a detailed account of the processing and analysing of data, obtained through video recording during reflective practitioner research. It sets out five stages in the analysis of video recordings of classroom interactions during a series of educational drama lessons: from decisions relating to the selection of data for close analysis, to the seeking of themes, and finally to the presentation of conclusions. The researcher adapted and synthesised several processes derived from discourse analysis (Wells, 2001; Spiers, 2004; Gee, 2005) to produce a range of instruments for use in transcription and analysis of verbal and non-verbal discourse. These include: a simple transcription key; classifications for verbal and non-verbal discourse; and a template for a transcription and analysis matrix
Detecting the Rise and Fall of 21 cm Fluctuations with the Murchison Widefield Array
We forecast the sensitivity with which the Murchison Widefield Array (MWA)
can measure the 21 cm power spectrum of cosmic hydrogen, using radiative
transfer simulations to model reionization and the 21 cm signal. The MWA is
sensitive to roughly a decade in scale (wavenumbers of k ~ 0.1 - 1 h Mpc^{-1}),
with foreground contamination precluding measurements on larger scales, and
thermal detector noise limiting the small scale sensitivity. This amounts
primarily to constraints on two numbers: the amplitude and slope of the 21 cm
power spectrum on the scales probed. We find, however, that the redshift
evolution in these quantities can yield important information about
reionization. Although the power spectrum differs substantially across
plausible models, a generic prediction is that the amplitude of the 21 cm power
spectrum on MWA scales peaks near the epoch when the intergalactic medium (IGM)
is ~ 50% ionized. Moreover, the slope of the 21 cm power spectrum on MWA scales
flattens as the ionization fraction increases and the sizes of the HII regions
grow. Considering detection sensitivity, we show that the optimal MWA antenna
configuration for power spectrum measurements would pack all 500 antenna tiles
as close as possible in a compact core. The MWA is sensitive enough in its
optimal configuration to measure redshift evolution in the slope and amplitude
of the 21 cm power spectrum. Detecting the characteristic redshift evolution of
our models will confirm that observed 21 cm fluctuations originate from the
IGM, and not from foregrounds, and provide an indirect constraint on the
volume-filling factor of HII regions during reionization. After two years of
observations under favorable conditions, the MWA can constrain the filling
factor at an epoch when ~ 0.5 to within roughly +/- 0.1 at 2-sigma.Comment: 14 pages, 9 figures, submitted to Ap
The initial mass function of early-type galaxies
We determine an absolute calibration of the initial mass function (IMF) of
early-type galaxies, by studying a sample of 56 gravitational lenses identified
by the SLACS Survey. Under the assumption of standard Navarro, Frenk & White
dark matter halos, a combination of lensing, dynamical, and stellar population
synthesis models is used to disentangle the stellar and dark matter
contribution for each lens. We define an "IMF mismatch" parameter
\alpha=M*(L+D)/M*(SPS) as the ratio of stellar mass inferred by a joint lensing
and dynamical models (M*(L+D)) to the current stellar mass inferred from
stellar populations synthesis models (M*(SPS)). We find that a Salpeter IMF
provides stellar masses in agreement with those inferred by lensing and
dynamical models (=0.00+-0.03+-0.02), while a Chabrier IMF
underestimates them (=0.25+-0.03+-0.02). A tentative trend is
found, in the sense that \alpha appears to increase with galaxy velocity
dispersion. Taken at face value, this result would imply a non universal IMF,
perhaps dependent on metallicity, age, or abundance ratios of the stellar
populations. Alternatively, the observed trend may imply non-universal dark
matter halos with inner density slope increasing with velocity dispersion.
While the degeneracy between the two interpretations cannot be broken without
additional information, the data imply that massive early-type galaxies cannot
have both a universal IMF and universal dark matter halos.Comment: 10 pages 4 figures. Resubmitted to ApJ taking into account referee's
comment
The Sloan Lens ACS Survey. VI: Discovery and analysis of a double Einstein ring
We report the discovery of two concentric Einstein rings around the
gravitational lens SDSSJ0946+1006, as part of the Sloan Lens ACS Survey. The
main lens is at redshift zl=0.222, while the inner ring (1) is at zs1=0.609 and
Einstein radius . The wider image separation () of the outer ring (2) implies that it is at higher redshift. Its
detection in the F814W filter implies zs2<6.9. The configuration can be well
described by a total density profile with
and velocity dispersion \sigma_{SIE}=287\pm5\kms. [...] We consider whether
this configuration can be used to constrain cosmological parameters exploiting
angular distance ratios entering the lens equations. Constraints for
SDSSJ0946+1006, are uninteresting due to the sub-optimal lens and source
redshifts. We then consider the perturbing effect of the mass associated with
Ring 1 building a double lens plane compound lens model. This introduces minor
changes to the mass of the main lens and allows to estimate the mass of Ring 1
(\sigma_{SIE,s1}=94\pm30\kms). We examine the prospects of doing cosmography
with a sample of 50 double lenses, expected from future space based surveys
such as DUNE or JDEM. Taking full account of the model uncertainties, such a
sample could be used to measure and with 10% accuracy, for a
flat cosmology
The Sloan Lens ACS Survey. VIII. The relation between environment and internal structure of early-type galaxies
We study the relation between the internal structure of early-type galaxies
and their environment using 70 strong gravitational lenses from the Sloan ACS
Lens Survey. The Sloan database is used to determine two measures of
overdensity of galaxies around each lens: the projected number density of
galaxies inside the tenth nearest neighbor (\Sigma_{10}) and within a cone of
radius one h^{-1} Mpc (D_1). Our main results are: 1) The average overdensity
is somewhat larger than unity, consistent with lenses preferring overdense
environments as expected for massive early-type galaxies (12/70 lenses are in
known groups/clusters). 2) The distribution of overdensities is
indistinguishable from that of "twin" non-lens galaxies selected from SDSS to
have the same redshift and stellar velocity dispersion \sigma_*. Thus, within
our errors, lens galaxies are an unbiased population, and the SLACS results can
be generalized to the overall population of early-type galaxies. 3) Typical
contributions from external mass distribution are no more than a few per cent,
reaching 10-20% (~0.05-0.10 external convergence) only in the most extreme
overdensities. 4) No significant correlation between overdensity and slope of
the mass density profile of the lens is found. 5) Satellite galaxies (those
with a more luminous companion) have marginally steeper mass density profiles
than central galaxies (those without). This result suggests that tidal
stripping may affect the mass structure of early-type galaxies down to kpc
scales probed by strong lensing, when they fall into larger structures
[ABRIDGED].Comment: ApJ, in press; minor changes with respect to v
- …